'\" t
.\"     Title: CREATE DOMAIN
.\"    Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: 2021
.\"    Manual: PostgreSQL 13.3 Documentation
.\"    Source: PostgreSQL 13.3
.\"  Language: English
.\"
.TH "CREATE DOMAIN" "7" "2021" "PostgreSQL 13.3" "PostgreSQL 13.3 Documentation"
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.SH "NAME"
CREATE_DOMAIN \- define a new domain
.SH "SYNOPSIS"
.sp
.nf
CREATE DOMAIN \fIname\fR [ AS ] \fIdata_type\fR
    [ COLLATE \fIcollation\fR ]
    [ DEFAULT \fIexpression\fR ]
    [ \fIconstraint\fR [ \&.\&.\&. ] ]

where \fIconstraint\fR is:

[ CONSTRAINT \fIconstraint_name\fR ]
{ NOT NULL | NULL | CHECK (\fIexpression\fR) }
.fi
.SH "DESCRIPTION"
.PP
\fBCREATE DOMAIN\fR
creates a new domain\&. A domain is essentially a data type with optional constraints (restrictions on the allowed set of values)\&. The user who defines a domain becomes its owner\&.
.PP
If a schema name is given (for example,
CREATE DOMAIN myschema\&.mydomain \&.\&.\&.) then the domain is created in the specified schema\&. Otherwise it is created in the current schema\&. The domain name must be unique among the types and domains existing in its schema\&.
.PP
Domains are useful for abstracting common constraints on fields into a single location for maintenance\&. For example, several tables might contain email address columns, all requiring the same CHECK constraint to verify the address syntax\&. Define a domain rather than setting up each table\*(Aqs constraint individually\&.
.PP
To be able to create a domain, you must have
USAGE
privilege on the underlying type\&.
.SH "PARAMETERS"
.PP
\fIname\fR
.RS 4
The name (optionally schema\-qualified) of a domain to be created\&.
.RE
.PP
\fIdata_type\fR
.RS 4
The underlying data type of the domain\&. This can include array specifiers\&.
.RE
.PP
\fIcollation\fR
.RS 4
An optional collation for the domain\&. If no collation is specified, the underlying data type\*(Aqs default collation is used\&. The underlying type must be collatable if
COLLATE
is specified\&.
.RE
.PP
DEFAULT \fIexpression\fR
.RS 4
The
DEFAULT
clause specifies a default value for columns of the domain data type\&. The value is any variable\-free expression (but subqueries are not allowed)\&. The data type of the default expression must match the data type of the domain\&. If no default value is specified, then the default value is the null value\&.
.sp
The default expression will be used in any insert operation that does not specify a value for the column\&. If a default value is defined for a particular column, it overrides any default associated with the domain\&. In turn, the domain default overrides any default value associated with the underlying data type\&.
.RE
.PP
CONSTRAINT \fIconstraint_name\fR
.RS 4
An optional name for a constraint\&. If not specified, the system generates a name\&.
.RE
.PP
NOT NULL
.RS 4
Values of this domain are prevented from being null (but see notes below)\&.
.RE
.PP
NULL
.RS 4
Values of this domain are allowed to be null\&. This is the default\&.
.sp
This clause is only intended for compatibility with nonstandard SQL databases\&. Its use is discouraged in new applications\&.
.RE
.PP
CHECK (\fIexpression\fR)
.RS 4
CHECK
clauses specify integrity constraints or tests which values of the domain must satisfy\&. Each constraint must be an expression producing a Boolean result\&. It should use the key word
VALUE
to refer to the value being tested\&. Expressions evaluating to TRUE or UNKNOWN succeed\&. If the expression produces a FALSE result, an error is reported and the value is not allowed to be converted to the domain type\&.
.sp
Currently,
CHECK
expressions cannot contain subqueries nor refer to variables other than
VALUE\&.
.sp
When a domain has multiple
CHECK
constraints, they will be tested in alphabetical order by name\&. (PostgreSQL
versions before 9\&.5 did not honor any particular firing order for
CHECK
constraints\&.)
.RE
.SH "NOTES"
.PP
Domain constraints, particularly
NOT NULL, are checked when converting a value to the domain type\&. It is possible for a column that is nominally of the domain type to read as null despite there being such a constraint\&. For example, this can happen in an outer\-join query, if the domain column is on the nullable side of the outer join\&. A more subtle example is
.sp
.if n \{\
.RS 4
.\}
.nf
INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));
.fi
.if n \{\
.RE
.\}
.sp
The empty scalar sub\-SELECT will produce a null value that is considered to be of the domain type, so no further constraint checking is applied to it, and the insertion will succeed\&.
.PP
It is very difficult to avoid such problems, because of SQL\*(Aqs general assumption that a null value is a valid value of every data type\&. Best practice therefore is to design a domain\*(Aqs constraints so that a null value is allowed, and then to apply column
NOT NULL
constraints to columns of the domain type as needed, rather than directly to the domain type\&.
.PP
PostgreSQL
assumes that
CHECK
constraints\*(Aq conditions are immutable, that is, they will always give the same result for the same input value\&. This assumption is what justifies examining
CHECK
constraints only when a value is first converted to be of a domain type, and not at other times\&. (This is essentially the same as the treatment of table
CHECK
constraints, as described in
Section\ \&5.4.1\&.)
.PP
An example of a common way to break this assumption is to reference a user\-defined function in a
CHECK
expression, and then change the behavior of that function\&.
PostgreSQL
does not disallow that, but it will not notice if there are stored values of the domain type that now violate the
CHECK
constraint\&. That would cause a subsequent database dump and reload to fail\&. The recommended way to handle such a change is to drop the constraint (using
\fBALTER DOMAIN\fR), adjust the function definition, and re\-add the constraint, thereby rechecking it against stored data\&.
.SH "EXAMPLES"
.PP
This example creates the
us_postal_code
data type and then uses the type in a table definition\&. A regular expression test is used to verify that the value looks like a valid US postal code:
.sp
.if n \{\
.RS 4
.\}
.nf
CREATE DOMAIN us_postal_code AS TEXT
CHECK(
   VALUE ~ \*(Aq^\ed{5}$\*(Aq
OR VALUE ~ \*(Aq^\ed{5}\-\ed{4}$\*(Aq
);

CREATE TABLE us_snail_addy (
  address_id SERIAL PRIMARY KEY,
  street1 TEXT NOT NULL,
  street2 TEXT,
  street3 TEXT,
  city TEXT NOT NULL,
  postal us_postal_code NOT NULL
);
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP
The command
\fBCREATE DOMAIN\fR
conforms to the SQL standard\&.
.SH "SEE ALSO"
ALTER DOMAIN (\fBALTER_DOMAIN\fR(7)), DROP DOMAIN (\fBDROP_DOMAIN\fR(7))
